To prevent accidental poisoning, you should not
a. mix chemicals in well ventilated areas
b. follow the instructions on the label
c. keep materials in unlabelled containers
d. use chemicals in well ventilated areas

To prevent accidental poisoning, you should not keep materials in unlabelled containers. The answer is letter C.  This is because unlabeled containers do not inform you what is inside the chemical, how dangerous it is and how to keep it. This practice is not acceptable because it will really cause harm. Using and mixing chemicals in well ventilated area are not prone to accident because the ventilation will prevent you from inhaling the harmful chemicals. And also, following the instructions in the label will help you how to handle a certain chemical.